Police were called to Tulse Hill station at about 6.30pm on Monday to reports of a man in possession of a knife.
A group of officers surrounded the man and pinned him to the ground. Pictures from the scene appeared to show a large machete on the ground nearby.
The man, 59, was arrested on suspicion of attempted murder and possession of an offensive weapon.
Conor Fortune said he was on a Thameslink train siting at Tulse Hill when he heard a male voice "shouting quite loudly".
The 39-year-old, from south-west London, said: "After several minutes of being sat on the platform, the driver announced that we were being held and she shut the doors as a precaution, and advised nobody to get off as there was 'a dangerous man on the platform'.
"She informed us the police had been alerted and were responding."
Brian Williams tweeted: "Excellent work by Met police at Tulse Hill. Illustrates the vital importance of having readily available taser equipped officers."
The incident is not believed to be terror related.
A British Transport Police (BTP) spokesman added: "Officers from BTP and the Metropolitan Police were called to Tulse Hill station after reports of a man in possession of a knife.
"Police officers were quickly on scene and detained a man on the platforms at the station. During the detainment a Taser device was discharged.
"The man is being taken to custody. There are no reported injuries."
Anyone who witnessed what happened is asked to contact BTP by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 478 of 14/01/2019.






Comment: What is happening in Britain? Random knife attacks seem to be in the news much more often these days:
- Man stabbed in the face on London Underground train - Latest attack in city's crime epidemic
- UK: Knifeman storms London Underground stabbing one man in chest meanwhile Birmingham knife attack leaves 3 injured
- Cyclist filmed in terrifying knife attack on motorist in South London
- Knife crime hits record high in England and Wales as violence soars, statistics reveal
Crime of all kinds has been documented to be increasing, and that could relate to the cuts to social services over the last decade which has led to a reduction in police numbers, see: London crime wave: Theft, burglary, rape, violent crime and homicide skyrocket